In recent years, a so-called fiber-reinforced resin material containing a matrix resin and reinforcing fibers such as carbon fibers has been attracted in the mechanical field. In joining of fiber-reinforced resin materials, which is required in manufacturing of components or structural bodies, a method of enhancing rigidity by forming a closed section has been suggested. When fiber-reinforced resin materials containing a thermoplastic resin as the matrix are joined, a mechanical fastening using bolts and nuts, rivet or the like, a chemical joining using an adhesive, and a thermal joining using an ultrasonic welding, a vibration welding, and the like have been suggested. Among them, caulking is a method of joining by deforming a tip end of a protrusion substance by heat, and a high joining strength in a peel direction may be obtained. Also, for the reason that various heating methods using heat or ultrasonic may be applicable when caulking, the caulking has been widely used in various industrial fields (see Patent Document 1).
When a caulking shape is broken in a peel direction, any of a base material breakage in which the base material is broken, a shaft part breakage in which the shaft is broken, a neck part breakage in which a base of umbrella part is broken, and an umbrella part breakage in which the inside of the umbrella part is broken parallel to the shaft occurs. When general resins are caulked, the shaft part breakage, the neck part breakage or the umbrella part breakage occurs depending on condition for caulking. Among them, the shaft part breakage and the neck part breakage are largely affected by a cross section of the shaft, i.e., shaft diameter. However, these are basically dependent on tensile modulus of a resin, and therefore the caulked part does not necessarily have a high joining strength. Conversely, it is necessary to make the shaft diameter thick in order to obtain a high joining strength, and it becomes a factor of limitation in design.
On the other hand, when the fiber-reinforced resin materials are caulked, since the fiber-reinforced resin material has excellent tensile strength than the general resins, the base material breakage or the umbrella part breakage occurs in many cases depending on caulking conditions. The base material breakage depends on strength or thickness of the base material. The umbrella part breakage is a breakage occurred by shear force at the inside of the umbrella part, the shear force applied in a parallel direction to the shaft, and it is important in design to impart excellent shear strength.